Process to prepare residual base oil

ABSTRACT

The present invention provides a process to prepare a residual base oil, which process at least comprises the following steps: (a) providing a mineral bright stock waxy raffinate; (b) combining the mineral bright stock waxy raffinate provided in step (a) with a catalytically dewaxed residual Fischer-Tropsch derived base oil to obtain a mixture; and (c) solvent dewaxing the mixture obtained in (b) to obtain a residual base oil, wherein the weight ratio of the mineral bright stock waxy raffinate to the catalytically dewaxed residual Fischer-Tropsch derived base oil in step (b) is in the range of from 75:25 to 35:65. In another aspect the present invention provides a residual base oil obtainable by the process.

The present invention relates to a process to prepare a residual base oil and to a residual base oil obtainable by this process.

It is known to use high viscosity (typically from 20 to 25 mm²/s) Fischer-Tropsch derived base oils to improve lubricating properties of base oils, such as pour point and cloud point. High viscosity base oils, also known as bright stock base oils, derived from Fischer-Tropsch synthesis, often show a hazy appearance that is typically due to the presence of a small quantity of microcrystalline wax particles.

WO 2007/003623 discloses a process to prepare a brightstock base oil blend comprising a paraffinic base oil base oil component having a viscosity at 100° C. of from 8 to 25 mm²/s, and a mineral derived residual and de-asphalted oil component in an amount from 40% to 99%, based on the total weight of the oil blend. In WO 2007/003623 catalytic dewaxing of a Fischer-Tropsch derived paraffinic base oil precursor component has resulted in a hazy Fischer-Tropsch derived paraffinic base oil component. The amount of wax present in the Fischer-Tropsch derived paraffinic base oil component, causing the hazy appearance, is not sufficient enough in order to solvent dewax the Fischer-Tropsch derived paraffinic base oil component in an efficient manner. Therefore, this hazy Fischer-Tropsch derived paraffinic base oil component is blended with the mineral derived residual at a temperature of greater than 50° C. to obtain a clear oil blend.

It is an object of the invention to provide a more efficient method for preparing a clear and bright residual base oil.

It is a further object of the present invention to provide an alternative method for preparing a clear and bright residual base oil.

Another object of the present invention is to prepare a clear and bright residual base oil using a large proportion of a high viscosity Fischer-Tropsch derived base oil.

Above or other objects are achieved according to the present invention by providing a process to prepare a residual base oil, the process at least comprising the following steps:

-   (a) providing a mineral bright stock waxy raffinate; -   (b) combining the mineral bright stock waxy raffinate provided in     step (a) with a catalytically dewaxed residual Fischer-Tropsch     derived base oil to obtain a mixture; and -   (c) solvent dewaxing the mixture obtained in (b) to obtain a     residual base oil.

It has now surprisingly been found according to the present invention that by solvent dewaxing a mixture comprising the mineral bright stock waxy raffinate and the catalytically dewaxed residual Fischer-Tropsch derived base oil, haze present in the mixture can be easily removed.

It has been found that at least 5 wt % of wax in the mineral bright stock waxy raffinate and the catalytically dewaxed residual Fischer-Tropsch derived base oil mixture is required in order to operate solvent dewaxing steps in an optimal manner. The amount of wax present in the catalytically dewaxed residual Fischer-Tropsch derived base oil alone is not sufficient enough in order to individually solvent dewax this base oil in an efficient manner. The amount of wax present in the catalytically dewaxed residual Fischer-Tropsch derived base oil is less than 0.1 wt %, based on the total amount of catalytically dewaxed residual Fischer-Tropsch derived base oil.

Solvent dewaxing of a mixture comprising the mineral bright stock waxy raffinate and the catalytically dewaxed residual Fischer-Tropsch derived base oil avoids the difficulty of removing haze from the catalytically dewaxed residual Fischer-Tropsch derived base oil alone, due to the presence of a sufficient amount of wax in the mixture (at least 5 wt. %), thereby resulting in increased efficiency.

A further advantage of the present invention is that a large amount of high viscosity Fischer-Tropsch derived base oil can be used in the mixture, yet a clear and bright high viscosity base oil is obtained.

Another advantage of the present invention is that a clear and bright residual base oil is obtained having a high viscosity, a low pour point and a low cloud point.

In step (a) of the process according to the present invention a mineral bright stock waxy raffinate is provided. Various processes to provide a mineral bright stock waxy raffinate are known in the art.

Suitably, the mineral bright stock waxy raffinate is prepared using the following steps:

-   (aa) providing a mineral derived vacuum residue; -   (bb) performing a de-asphalting step on the mineral derived vacuum     residue to obtain a de-asphalted oil; and -   (cc) solvent extracting the de-asphalted oil to obtain a residual     aromatic extract and the mineral bright stock waxy raffinate.

In step (aa) a mineral derived vacuum residue is provided. Mineral derived vacuum residue as provided in step (aa) may be a residue bottom fraction of a vacuum distillation of a crude petroleum feed. A preferred property of the mineral derived vacuum residue is that 90 wt. % boils above 500° C., more preferably above 520° C.

In step (bb) a de-asphalting step on the mineral derived vacuum residue is performed to obtain a de-asphalted oil. A de-asphalted oil is the product of a de-asphalting process step wherein asphalt is removed from a mineral derived vacuum residue. De-asphalting processes are well known and for example described in Lubricant base oil and wax processing, Avilino Sequeira, Jr., Marcel Dekker, Inc, New York, 1994, ISBN 0-8247-9256-4, pages 53-80. Typically, the de-asphalting process utilizes a light hydrocarbon liquid solvent, for example propane, for asphalt compounds. A preferred property of the de-asphalted oil is that 90 wt. % boils above 470° C., more preferably above 490° C.

In step (cc) the de-asphalted oil is subjected to solvent extraction to obtain a residual aromatic extract and a mineral bright stock waxy raffinate.

The de-asphalted oil is subjected to a solvent extraction process in order to remove some of the aromatic compounds. Solvent extraction processes are known in the art and therefore not discussed here in detail. Typical solvent extraction processes are for example described in Chapter 5 of “Lubricant base oil and wax processing”, Avilino Sequeira, Jr., Marcel Dekker, Inc, New York, 1994, ISBN 0-8247-9256-4.

Usually, following solvent extraction of the de-asphalted oil, the solvent is stripped off from the mineral bright stock waxy raffinate, and a residual aromatic extract, containing mostly aromatics, some dissolved mineral bright stock waxy raffinate and some wax, is sent to a recovery unit. This process is for example described in U.S. Pat. No. 4,592,832.

Mineral bright stock waxy raffinate as provided in step (a) is a resulting extracted de-asphalted oil phase, containing mainly non-saturated hydrocarbon molecules, as obtained in step (cc). Suitably, the amount of non-saturated hydrocarbon molecules is between 50 to 60 wt %, based on the total amount of mineral bright stock waxy raffinate.

Suitably, the mineral bright stock waxy raffinate as provided in step (a) has a kinematic viscosity at 40° C. according to ASTM D-445 of from 200 to 700 mm²/s, preferably from 300 to 600 mm²/s, more preferably from 400 to 600 mm²/s, and most preferably from 500 to 600 mm²/s.

Also, the kinematic viscosity at 100° C. of the mineral bright stock waxy raffinate according to ASTM D-445 is suitably from 20 to 50 mm²/s, preferably from 30 to 50 mm²/s, and more preferably from 30 to 40 mm²/s. Typically, the viscosity index of the mineral bright stock waxy raffinate is between 50 and 150, preferably between 70 and 120, more preferably between 80 and 100.

In step (b) the mineral bright stock waxy raffinate provided in step (a) is combined with a catalytically dewaxed residual Fischer-Tropsch derived base oil to obtain a mixture.

In an alternative embodiment of the process according the present invention, a part of the de-asphalted oil obtained in step (b), which part is not subjected to the solvent extraction step (cc), is combined with a catalytically dewaxed residual Fischer-Tropsch derived base oil to obtain a mixture.

Suitably, at least 20 wt. % of the de-asphalted oil is combined with the catalytically dewaxed residual Fischer-Tropsch derived base oil to obtain a mixture, preferably at least 30 wt % of the de-asphalted oil, and more preferably at least 50 wt % of the de-asphalted oil.

The catalytically dewaxed residual Fischer-Tropsch derived base oil as used in step (b) is derived from a Fischer-Tropsch process. Fischer-Tropsch derived base oil is known in the art. By the term “Fischer-Tropsch derived” is meant that a base oil is, or is derived from, a synthesis product of a Fischer-Tropsch process. A Fischer-Tropsch derived base oil may also be referred to as a GTL (Gas-to-Liquids) base oil.

Solvent dewaxing the catalytically dewaxed residual base oil will be quite difficult because the amount of wax is so low, but more importantly it will be very difficult to filter on a filter cloth because the wax has the consistency of “chewing gum”. This consistency of a “chewing gum” is produced by the catalytically dewaxing step, which step is necessary to obtain a Fischer-Tropsch derived high viscosity base oil with a desired pour point. The Fischer-Tropsch product from which the catalytically dewaxed residual base oil is derived has an unfavorable pour point as well as being heavy and therefore could not be easily exported from a Fischer-Tropsch production facility.

The preparation of the catalytically dewaxed residual Fischer-Tropsch derived base oil as used in step (b) has been described in e.g. WO 2007/003623 and in WO 2007/003617.

Catalytically dewaxed Fischer-Tropsch derived base oils of the present invention typically are a heavy base oil component comprising carbons of up to around C65, typically in the range of between at least C20 and at most C65, and a wax component of between at least 0.001 wt. % and at most than 5 wt. %, typically at most than 1 wt. %, suitably at most than 0.1 wt. %. Typically, the amount of wax component present in the catalytically dewaxed residual Fischer-Tropsch derived base oil depends on the severity of the pour point.

The hazy catalytically dewaxed residual Fischer-Tropsch derived base oils of the invention are visibly at least partially or completely opaque at ambient temperature. Hence, it will be apparent to the skilled person that the catalytically dewaxed residual Fischer-Tropsch derived base oils utilised in the present invention comprise sufficient additional heavy components (such as wax) to impart a visible haze to the appearance of the oil. Typically, the amount of wax, which impart a visible haze, is between 0.01 wt. % to 0.1 wt. % based on the total amount of catalytically dewaxed residual Fischer-Tropsch derived base oil. As such, the base oils of the present invention would not be described conventionally as ‘clear’ or ‘bright’.

Suitably, the catalytically dewaxed residual Fischer-Tropsch derived base oil as used under step (b) typically has a kinematic viscosity at 40° C. according to ASTM D-445 of above 80 mm²/s, suitably above 100 mm²/s. Typically, the kinematic viscosity at 40° C. of the catalytically dewaxed Fischer-Tropsch derived base oil according to the present invention is below about 300 mm2/s.

Also, the kinematic viscosity at 100° C. according to ASTM D-445 of the Fischer-Tropsch derived base oil is from 12 to 50 mm²/s, preferably from 15 to 35 mm²/s, more preferably from 18 to 30 mm²/s and most preferably from 18 to 25 mm²/s.

The catalytically dewaxed residual Fischer-Tropsch derived base oil preferably has a pour point according to ASTM D-5950 of below 0° C., more preferably below −5° C., and most preferably below −10° C.

Typically, a hazy paraffinic base oil has a cloud point of 15° C. and above. Preferably, the catalytically dewaxed residual Fischer-Tropsch derived base oil has a cloud point according to ASTM D-5950 of above 15° C., more preferably above 20° C., more preferably above 25° C. and most preferably above 30° C.

Suitably, the mixture obtained in step (b) comprises from 10 to 80 wt. %, preferably from 20 to 70 wt. %, more preferably from 25 to 65 wt. % of the catalytically dewaxed residual Fischer-Tropsch base oil.

Preferably, the weight ratio of the mineral bright stock waxy raffinate to the catalytically dewaxed residual Fischer-Tropsch base oil in step (b) of the present process is in the range of from 75:25 to 35:65, more preferably in the range of from 65:35 to 35:65, most preferably 55:45 to 35:65.

The mixture obtained in step (b) will typically comprise a fraction in which 50% boils above 450° C., preferably in which 50% boils above 550° C. It is this high boiling fraction, which will yield the viscous base oils.

Typically, the mixture obtained in step (b) will comprise a fraction which 90% boils below 820° C.

The wax content in the mixture is preferably below 20 wt. %, more preferably below 10 wt. %. The lower limit is preferably above 4 wt. %.

In step (c) a solvent dewaxing step on the mixture obtained in step (b) is performed to obtain a residual base oil.

Solvent dewaxing processes are known in the art and therefore not described here in detail. Typical solvent dewaxing processes are for example described in Lubricant Base Oil and Wax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc., New York, 1994, Chapter 7.

In a preferred embodiment the process according to the present invention, comprises a further step separating heteroatoms from the residual base oil obtained in step (c).

Preferably, the residual base oil obtained in step (c) comprises sulphur and nitrogen compounds containing in amounts of less than 50 ppmw, more preferably less than 20 ppmw, yet more preferably less than 10 ppmw. Most preferably it will comprise sulphur and nitrogen at levels generally below the detection limits, which are currently 5 ppm for sulphur and 1 ppm for nitrogen when using for instance X-ray or Antek Nitrogen tests for determination. However, sulphur may be introduced through the use of sulphided hydrocracking/hydrodewaxing and/or sulphided catalytic dewaxing catalysts. If desired a final finishing treatment may be performed in order to separate the sulphur and nitrogen compounds from the residual base oil. Examples of suitable finishing treatments are so-called sulfuric acid treating processes, hydrofinishing or hydrogenation processes and adsorption processes. Sulfuric acid treating is for example described in General Textbook “Lubricant Base Oil and Wax Processing”, Avilino Sequeira, Jr, Marcel Dekker Inc., New York, 1994, Chapter 6, pages 226-227.

Hydrofinishing is suitably carried out at a temperature between 180 and 380° C., a total pressure of between 10 to 250 bar and preferably above 100 bar and more preferably between 120 and 250 bar. The WHSV (Weight hourly space velocity) ranges from 0.3 to 2 kg of oil per litre of catalyst per hour (kg/l·h).

In a further aspect the present invention provides a residual base oil obtainable by the process according to the present invention.

The haze free residual base oil will preferably have a kinematic viscosity at 100° C. according to ASTM D-445 of from 12 to 50 mm²/s, preferably from 15 to 35 mm²/s, more preferably from 18 to 30 mm²/s and most preferably from 18 to 25 mm²/s.

The viscosity index of the residual base oil is preferably greater than 95, preferably greater than 100.

The pour point of the residual base oil according to ASTM D-5950 is below −5° C., preferably below −10° C., and more preferably below −15° C.

A haze free base oil can also be determined by its cloud point. The residual base oil preferably has a cloud point according to ASTM D-5950 of below −10° C., more preferably below −15° C. and most preferably below −20° C.

FIG. 1 schematically shows a process scheme of the process according to the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

The process scheme is generally referred to with reference numeral 1.

From a crude petroleum feed 10 a mineral derived vacuum residue 20 is recovered by vacuum distillation in a vacuum distillation unit 2. The mineral derived vacuum residue 20 is de-asphalted in reactor 3 to obtain a de-asphalted oil 30. From the de-asphalted oil 30 a residual aromatic extract 40 and a mineral bright stock waxy raffinate 50 are extracted in reactor 4. The residual aromatic extract 40 and the solvent are led out of the reactor 4 as stream 40 to recovery unit 5. A stream comprising a hazy catalytically dewaxed residual Fischer-Tropsch derived base oil 60 is combined with the mineral bright stock waxy raffinate 50 upstream of reactor 6 wherein the combined streams are solvent dewaxed to obtain a residual base oil 70 and a waxy product (mixed slack waxes) 80. Sulphur and nitrogen in the form of hydrocarbon compounds are separated from the residual base oil 70, by a final finishing treatment in reactor 7.

The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the present invention in any way.

EXAMPLE 1 Preparation of a Mineral Bright Stock Waxy Raffinate

A mineral bright stock waxy raffinate was obtained from Shell Pernis Refinery (Pernis, Netherlands). The mineral brightstock waxy raffinate was prepared by performing a de-asphalting step on a mineral derived vacuum residue (high sulphur Middle East crude oil, e.g. Arab Light). The obtained de-asphalted oil was extracted with furfural using a solvent to fresh feed ratio that is on average 3.8 wt/wt (min. 2.4 wt/wt, max. 6.3 wt/wt) to obtain a residual aromatic extract and a mineral bright stock waxy raffinate. Furfural solvent extraction processes are known in the art and for example described in Chapter 5 of “Lubricant base oil and wax processing”, Avilino Sequeira, Jr., Marcel Dekker, Inc, New York, 1994, ISBN 0-8247-9256-4. The properties of the obtained mineral bright stock waxy raffinate are listed in Table 1.

TABLE 1 Mineral bright stock waxy raffinate Kinematic viscosity at 40° C. 517.5 according to ASTM D-445 [cSt] Kinematic viscosity at 100° C. 32.57 according to ASTM D-445 [cSt] VI 94

Preparation of a Catalytically Dewaxed Residual Fischer-Tropsch Derived Base Oil

The catalytically dewaxed residual Fischer-Tropsch derived base oil was obtained by the process as described in Example 1 of WO 2007/003623. The properties of the obtained catalytic dewaxed residual are listed in Tables 2 and 3.

TABLE 2 Catalytically dewaxed residual Fischer- Tropsch derived base oil Kinematic viscosity at 25.22 100° C. According to ASTM D445 [mm²/s] Kinematic viscosity at 92.92 60° C. According to ASTM D445 [mm²/s] VI according to ASTM 140 D2270 content of aromatics 2.8 According to IP 368 [% m/m] content of saturates 95.7 according to IP 368 [% m/m] content of recovery 98.5 according to IP 368 [% m/m] Pour point according to −6 ASTM D5950 [° C.] Density according to 840.2 IP365/97 [Kg/m³] Flash point according to 270.5 ASTM D-93 [° C.]

TABLE 3 Feed to catalytic dewaxing IBP (° C.) 448 Wt. % recovered at 476.0° C. 5 497.5° C. 10 514.0° C. 15 527.5° C. 20 539.5° C. 25 550.5° C. 30 562.0° C. 35 574.0° C. 40 585.5° C. 45 597.5° C. 50 609.5° C. 55 622.5° C. 60 636.0° C. 65 651.0° C. 70 666.5° C. 75 684.0° C. 80 702.5° C. 85 724.0° C. 90 750.0° C. 95

Preparation of a Residual Base Oil

Several mixtures were prepared by blending the residual Fischer-Tropsch derived base oil and mineral bright stock waxy raffinate in different ratios (see Table 4: Experiments A, B, C and D) together at 60° C. for 15 minutes.

The obtained mixtures were subjected to the following conventional solvent dewaxing step.

A weighed amount of the obtained mixtures (120 g) was dissolved in a specified amount of a mixture of toluene and methyl ethyl ketone (50/50) at the ratio 4/1. The solution was cooled to the required dewaxing temperature (−20° C.), allowing the slack waxes to precipitate, and the liquid phase was separated by filtration using a supported filter paper, (Whatman No. 41 catalogue number: 1441090, Whatman Division of GE Health Care). Solvent was removed from the residual base oil obtained under vacuum to less than 1000 ppm.

The properties of the obtained residual base oils are listed in Table 5.

TABLE 4 Experiment Experiment Experiment Experiment A B C D Amount of 75 65 55 35 mineral bright stock waxy raffinate [wt. %] Amount of 25 35 45 65 Cat. dewaxed residual FT derived base oil [wt. %]

TABLE 5 Properties of Residual Experiment Experiment Experiment Experiment base oils A B C D Kinematic 26.82 25.38 23.72 21.27 viscosity at 100° C. according to ASTM D-445 [cSt] Kinematic 326.1 284 246.9 194.3 viscosity at 40° C. according to ASTM D-445 [cSt] VI 109 115 120 130 Pour point −15 −15 −18 −27 according to ASTM D-5950 (° C.) Cloud point −16 −16 −14 −13 according to ASTM D-5950 (° C.) Visual Clear and Clear and Clear and Clear and Appearance bright bright bright bright

Discussion

The results in Table 5 show that solvent dewaxing mixtures as obtained in Experiments A to D according to the present invention results in clear and bright residual base oils. Furthermore, by adding a large proportion of catalytic dewaxed residual Fischer-Tropsch derived base oil in the mixture (see for example Experiment D) still clear and bright base oils were obtained. In addition, by adding a large proportion of catalytically dewaxed residual Fischer-Tropsch base oil in the mixtures (see Experiment A to D in Table 4) improvement of the pour points of the residual base oils (see Table 5) are observed compared to the pour point of catalytic dewaxed residual Fischer-Tropsch derived base oil (see Table 2). 

1. A process to prepare a residual base oil, the process comprising the steps of: (a) providing a mineral bright stock waxy raffinate; (b) combining the mineral bright stock waxy raffinate provided in step (a) with a catalytically dewaxed residual Fischer-Tropsch derived base oil to obtain a mixture; and (c) solvent dewaxing the mixture obtained in (b) to obtain a residual base oil, wherein the weight ratio of the mineral bright stock waxy raffinate to the catalytically dewaxed residual Fischer-Trospch derived base oil in step (b) is in the range of from 75:25 to 35:65.
 2. The process according to claim 1, wherein the mineral bright stock waxy raffinate is prepared using the following steps: (aa) providing a mineral derived vacuum residue; (bb) performing a de-asphalting step on the mineral derived vacuum residue to obtain a de-asphalted oil; and (cc) solvent extracting the de-asphalted oil to obtain a residual aromatic extract and the mineral bright stock waxy raffinate.
 3. The process according to claim 1, wherein the mineral bright stock waxy raffinate in step (a) has a kinematic viscosity at 100° C. from 20 to 50 mm²/s, preferably from 30 to 50 mm²/s, more preferably from 30 to 40 mm²/s.
 4. The process according to claim 1, wherein the catalytically dewaxed residual Fischer-Tropsch base oil in step (b) has a kinematic viscosity at 100° C. from 12 to 50 mm²/s.
 5. The process according to claim 1, wherein the catalytically dewaxed residual Fischer-Tropsch derived base oil in step (b) has a pour point of below 0° C.
 6. The process according to claim 1, wherein the catalytically dewaxed residual Fischer-Tropsch derived base oil in step (b) has a cloud point of above 15° C.
 7. The process according to claim 1, wherein the mixture obtained in step (b) comprises from 10 to 80 wt. %.
 8. A residual base oil obtainable by the process according to claim
 1. 9. The residual base oil according to claim 8, having a kinematic viscosity at 100° C. from 12 to 50 mm²/s.
 10. The residual base oil according to claim 8, having a viscosity index of greater than
 95. 11. The residual base oil according to claim 8, having a pour point of below −5° C.
 12. The residual base oil according to claim 8, having a cloud point of below −10° C. 